A read-only memory (ROM) is a type of memory in which data can be permanently stored, for example, by blowing metallic links during programming thereof. Such type of memory can be conventionally accessed at specific locations to read the programmed contents thereof. A much more versatile type of ROM is the electrically programmable read-only memory (EPROM) which can be electrically programmed to store data. Some versions of these devices store data permanently; others, however, can be erased with ultraviolet light or an electrical current.
Buried bit line EPROM or EEPROM arrays are known which utilize buried, doped lines, usually n+ conductivity doped crystalline silicon under thick oxide regions as bit lines for the array. The doped lines then act as the source/drain regions of the transistors of the memory cells. A typical EPROM of the advanced contactless array type is disclosed, for example in U.S. Pat. No. 4,698,900, issued Oct. 13, 1987.
EPROM cells in which tunnel windows are cut through the thick oxide layers that are formed over the buried bit lines have also been built. In these windows, the oxide thickness above the buried n+ regions is thin enough to allow the well known tunneling phenomena to occur for writing and/or erasing to the storage transistors.
However, several problems exist with the current state of the art. Current flash EEPROMs using buried bit lines employ the tunnel window at the field oxide and buried oxide interface. It is becoming more and more difficult to scale the filed oxide due to moat encroachment. This problem increases the cell size and therefore limits the number of cells which can be built on a chip.
Another problem exists with the yield and reliability of devices built using prior art fabrication methods. Due to the relatively large size of the tunnel windows built using prior art methods, yield is low and reliability is poor at the thin oxide. A method which greatly reduces the surface area of tunnel window is desirable. In addition, prior art methods usually employ wet etch techniques to open the patterned tunnel windows prior to growing the tunnel oxide. Because of the isotropic nature of wet etches, the tunnel windows are enlarged. Anisotropic dry etch methods could also be used to open the tunnel window but are not used because dry etch introduces risk of damaging and contaminating the silicon surface.
Accordingly, improvements which overcome any or all of the problems are presently desirable.